In general, DC/AC converter circuits, i.e. DC to AC voltage converters, are devices operative to output a sinusoidal voltage having predetermined, but variable amplitude and frequency according to necessity. Their operation is based on the controlled switching of a DC voltage input to an inverter incorporating controlled electronic switches.
Converters have been known which are operative to supply systems wherein the load may significantly affect the converter output quantities, and which are feedback regulated to avoid altering the sinusoidal output waveform. An embodiment of a feedback regulated converter operated in the so-called sliding mode is disclosed in Patent Application No. MI92A001290 filed by this Applicant on May 25, 1992. In this prior art, the inverter output voltage and current are compared with a sinusoidal reference voltage and a sinusoidal reference current, respectively, and the result of the comparison, i.e. the error signal, is used to automatically set that sequence and those switching times for the inverter switches which are most effective to keep the output quantities at the desired values.
In many practical applications of the converters, a transformer must be used to both output a voltage with an amplitude other than that of the inverter own output voltage, usually a larger one, and to ensure galvanic isolation of the DC voltage source from the load.
In transformer-type converters which are feedback regulated by measuring output quantities from the transformer secondary, there may arise problems from saturation of the transformer core. In fact, due to asymmetric or uneven operation of the inverter--such as may be brought about by slight physical differences in the electronic switches--or to intentional or unintentional time lags in the make and break commands to the various switches, a DC voltage component may appear at the transformer primary. Since the feedback system cannot sense this component due to the measurements being taken at the secondary circuit, the DC component will result in increased magnetization current, and attendant likelihood of the core becoming saturated.
To attenuate this effect, expedients have been proposed circuit-wise for raising the DC voltage input to the inverter in a gradual fashion. However, this prior approach, while avoiding core saturation from turn-on phenomena by virtue of the core magnetization occurring therein at a slowed rate, falls short of solving the problem which is primarily tied, as mentioned, to structural asymmetries of the inverter.